Which of the following compounds undergoes nucleophilic substitution reaction most easily?

In qualitative analysis, the metals of group I can be separated from other ions by precipitating them as chloride salts. A solution initially contains Ag⁺ and pb²⁺ at a concentration of 0.10 M. Aqueous HCl is added to this solution until the Cl^$$-$$ concentration is 0.10 M. What will the concentrations of Ag⁺ and Pb²⁺ be at equilibrium ?

(K_sp for AgCl = 1.8 $$ \times $$ 10^$$-$$10, K_sp for PbCl₂ = 1.7 $$ \times $$ 10^$$-$$5)

Consider the following processes :

	H (kJ/mol)
1/2A $$ \to $$ B	+150
3B  $$ \to $$ 2C + D	-125
E + A  $$ \to $$ 2D	+350

For B + D $$ \to $$ E + 2C, $$\Delta $$H will be

Which of the following structures is the most preferred and hence of lowest energy for SO₃?